In recent years, metallic nanoparticles and metal clusters have been embraced by industrial sectors due to their applications in the field of electronic storage systems, optics and medical imaging, biotechnology, magnetic separation and pre-concentration of target analytes, targeted drug delivery, and vehicles for gene and drug delivery. Generally, a nanoparticle (or nanopowder or nanocluster or nanocrystal) is a small object that behaves as a whole unit in terms of its transport and properties. Typically, such particles have at least one dimension less than 100 nm. Generally, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. A metal cluster contains a group of two or more metal atoms in which direct and substantial metal-metal bonding is present.
The adoption of this technology is hindered by the difficulties associated with producing metallic nanoparticles and metal clusters. For example, there is no existing process that allows for the direct production of metallic nanoparticles from a starting metal. The typical production process employs a strong acid to oxidize the starting metal to produce a precursor metallic salt. That metallic salt is then purified and isolated. The purified and isolated metallic salt is then chemically reduced using reducing agents (such as NaBH4) in bulk solution in the presence of capping ligands to produce the metal nanoparticles. The existing process is complex, requiring multiple separate reactions, in separate reaction steps, and is costly.
Metal clusters are traditionally produced in vacuum using laser ablation. A laser is applied to a metal target to vaporize the metal under vacuum, thereby producing metal cluster ions. The metal cluster ions are then contacted to a chemical reducing agent to reduce the metal cluster ions to neutral metal clusters. The existing process is costly, requiring expensive lasers and vacuum chambers, and also requires chemical reducing agents. Those processes that do operate at atmospheric pressure are difficult to control.